feature

Low Noise: 6 nV/√Hz; Low Offset Voltage: 100µV Max; Low Input Bias Current: 10pa Max; Fast Settling: 600 ns to 0.01%; Low Distortion; Unity Gain Stable; No Phase Reversal; Dual Supply Operation : ±5 V to ±13 V.

application

Photodiode amplifiers; instrumentation; sensors and controls; high performance filters; fast precision integrators high performance audio.

General Instructions

The AD8610/AD8620 are very high precision JFET input amplifiers with ultralow bias voltage and drift, very low input voltage and current noise, very low input bias current, and wide frequency band. Unlike many JFET amplifiers, the AD8610/AD8620 input bias current is low over the entire operating temperature range. The AD8610/AD8620 are stable with capacitive loads in excess of 1000 pf without changing unity gain; larger capacitive loads are easily driven at higher noise gains. The AD8610/AD8620 can swing within 1.2V of the supply even under a 1kΩ load, maximizing dynamic range even with limited supply voltages. The output transitions in an inverting or non-inverting gain configuration of 50 V/µs and settles to 0.01% accuracy in less than 600 ns. The AD8610/AD8620 feature high input impedance, high accuracy, and extremely high output drive capability, making them ideal amplifiers for driving high performance ADC inputs and buffering DAC converter outputs.

Applications for the AD8610/AD8620 include electronic instrumentation; ATE amplification, buffering, and integrator circuits; CAT/MRI/ultrasound medical instrumentation; instrumentation-quality photodiode amplification; fast precision filters (including PLL filters); and high-quality audio.

The AD8610/AD8620 are fully specified over the extended industrial temperature range (-40°C to +125°C). The AD8610 is available in narrow 8-lead SOIC and tiny 8-lead MSOP surface mount packages. The AD8620 is available in a narrow 8-lead SOIC package. 8-lead MSOP packaged devices are only available in tape and reel.

Function description

The AD8610/AD8620 are fabricated on Analog Devices' XFCB (Extra Fast Complementary Bipolar) process. The xfcb is fully insulated (di) and used in combination with n-channel jfet technology and thin film resistors (which can be trimmed) to create a jfet input amplifier. Dielectric isolated npn and pnp transistors fabricated on xfcb have f > 3ghz. Low tc thin film resistors allow very precise compensation voltage and compensation voltage temperature coefficient trimming. These process breakthroughs have allowed analog IC designers to create an amplifier with more than 50 percent faster switching speeds and more than 50 percent higher bandwidth while consuming half the current of its closest competitor. The AD8610/AD8620 are unconditionally stable in all gains, even with capacitive loads well over 1nF. The AD8610B stage achieves less than 100µV of bias and 1µV/°C of bias drift, figures typically associated with very high precision bipolar input amplifiers. The AD8610 is available in tiny 8-lead MSOP and narrow 8-lead SOIC surface mount packages and is fully specified for supply voltages from ±5.0 V to ±13 V. A very wide specified temperature range (up to 125°C) guarantees excellent performance in systems with little or no active cooling.

The unique input architecture of the AD8610/AD8620 provides very low input bias current and very low input bias voltage. Low power consumption minimizes die temperature and maintains extremely low input bias current. Unlike many competing JFET amplifiers, the AD8610/AD8620 input bias current is low even at high temperatures. Typical bias current is less than 200pa at 85°C. The gate current of the jfet doubles every 10°C, causing the input bias current to increase with temperature. Pay special attention to the layout of the PC board to minimize leakage current between PCB traces. Improper layout and board handling can generate leakage currents that exceed the bias currents of the AD8610/AD8620.

Power consumption

In new designs, a major advantage of the AD8610/AD8620 is power-saving capabilities. The low power consumption of the AD8610/AD8620 makes it more attractive in portable instruments and high-density systems, simplifying thermal management and reducing power performance requirements. Compare the power dissipation of the AD8610 and OPA627 in Figure 43.

Driving large capacitive loads

The AD8610/AD8620 have excellent capacitive load drive capability and can safely drive up to 10nF when operating from ±5.0V supplies. Figure 44 and Figure 45 compare the AD8610/AD8620 driving a parallel 10 kΩ resistor and 10000 pf capacitor at the output with the OPA627 with a square wave input frequency set to 200 kHz. The AD8610/AD8620 ring much less than the OPA627 under heavy capacitive loads.

The AD8610/AD8620 can drive larger capacitors without any external compensation. Although the AD8610/AD8620 are stable with very large capacitive loads, keep in mind that this capacitive loading limits the bandwidth of the amplifier. Heavy capacitive loads also increase the amount of overshoot and ringing at the output. Figure 47 and Figure 48 show the AD8610/AD8620 and OPA627 in an irreversible gain of +2 driving a 2µf capacitive load. The ringing on the opa627 is bigger and 10 times longer than the ad8610/ad8620.

Slew rate (unity gain inversion vs non-inversion)

Amplifiers typically have faster slew rates in an inverting unity-gain configuration due to the absence of differential input capacitors. Figure 49 through Figure 52 show the performance of the AD8610/AD8620, which has a unity gain of -1 compared to the OPA627. The slew rates of the AD8610/AD8620 are more symmetrical, with much cleaner positive and negative transitions than the OPA627.

The AD8610/AD8620 have a very fast slew rate of 60 V/µs, even at a non-inverting gain of +1. This is the most difficult condition to impose on any amplifier, as the amplifier's input common-mode capacitance usually makes its SR appear worse. The slew rate of an amplifier varies according to the voltage difference between its two inputs. In order to observe the maximum sr, one must ensure that the voltage difference between the inputs is about 2v. This is required for almost all JFET op amps so that one side of the op amp's input circuit is completely disconnected, maximizing the charge and discharge currents of the internal compensation capacitors. Lower differential drive voltages produce lower slew rate readings. If a JFET input op amp operates at a gain of +100 V = 100 mV at unity gain of V = 10 V, an op amp with a slew rate of 60 V/μS can slew at 20 V/μS. In the

At unity gain of +1, the AD8610/AD8620 have twice the slew rate of the OPA627 (see Figure 53 and Figure 54).

The slew rate of the amplifier determines its maximum frequency of response to large signal inputs. This frequency (called the full power bandwidth or fpbw) can be used to calculate the value of a given distortion (eg 1%) according to the formula:

Input overvoltage protection

When the input of the amplifier is driven below or above V by more than one V, large current flows from the substrate to the input pins through the negative supply (V-) or the positive supply (V+), respectively, and can damage the device. If the input source can supply more current than the diode's maximum forward current (greater than 5mA), a series resistor can be added to protect the input. Due to its extremely low input bias and bias current, a large series resistor can be placed in front of the AD8610/AD8620 input to limit the current to a level below damage. A series resistance of 10 kΩ produces an offset of less than 25 μV. This 10 kΩ allows input voltages to exceed 5 V from either supply. Thermal noise from the resistors adds 7.5 nV/√Hz to the AD8610/AD8620 noise. For the AD8610/AD8620, a differential voltage equal to the supply voltage does not cause any problems (see Figure 55). In this case, note that the high breakdown voltage of the input fet eliminates the need to include clamping diodes between the amplifier's inputs, which is a must for many precision op amps. Unfortunately, clamping diodes severely interfere with many application circuits, such as precision rectifiers and comparators. The AD8610/AD8620 do not suffer from these limitations.

No phase reversal

Many amplifiers exhibit erroneous behavior when one or both inputs are forced beyond the input common-mode voltage range. Phase inversion is represented by the transfer function of the amplifier, effectively reversing its transfer polarity. In some cases, this can lead to lockups in the servos or even equipment damage, and can result in permanent damage or no recoverable parameter movement in the amplifier itself. Many amplifiers have compensation circuits to counteract these effects, but some amplifiers only respond to inverting inputs. The AD8610/AD8620 are designed to prevent phase reversal when one or both inputs are forced beyond their input common-mode voltage range.

THD reading vs common mode voltage

The AD8610/AD8620 have THD well below 0.0006% at any load below 600Ω. In terms of distortion, the AD8610 outperforms the OPA627, especially at frequencies above 20kHz.

Settling time

As shown in Figure 60, the AD8610/AD8620 have very fast settling times, even with very small error bands. The AD8610/AD8620 are configured in an inverting gain of +1 with 2 kΩ input and feedback resistors. Monitor the output with a 10 × 10 MΩ 11.2 pf oscilloscope probe.

As shown in Figure 62, the AD8610/AD8620 maintain this fast settling time when loaded with a large capacitive load.

output current capability

The AD8610/AD8620 can drive very heavy loads due to their high output current. It is capable of generating or sinking 45 mA at ±10 V outputs. Short circuit currents are high, the part is capable of sinking about 95mA and drawing over 60mA when operating from ±13 V supplies. Figure 64 and Figure 65 compare the output voltage and load current of the AD8610/AD8620 and the OPA627.

Although the operating conditions (±13 V) imposed on the AD8610/AD8620 are not as favorable as the OPA627 (±15 V), it can be seen that the AD8610/AD8620 have better drive capability (lower supply headroom) for a given load current ).

Operates on supplies greater than ±13 V

The maximum operating voltage of the AD8610/AD8620 is specified as ±13 V. Inexpensive LDOs can provide ±12 V from a nominal ±15 V supply when ±13 V is not readily available.

Input Bias Voltage Adjustment

The offset of the AD8610 is so small that no additional offset adjustment is usually required. However, offset adjustment pins can be used as shown in Figure 66 to further reduce the dc offset. Using resistors in the 50 kΩ range, the offset trim range is ±3.3 mV.

Programmable Gain Amplifier (PGA)

The combination of low noise, low input bias current, low input bias voltage, and low temperature drift make the ad8610/ad8620 the perfect solution for programmable gain amplifiers. PGAs are often used immediately after the sensor to increase the dynamic range of the measurement circuit. Historically, the large on-resistance of the switch (plus the large current of the amplifier) created a large DC offset in the PGA. Recent and improved monolithic switches and amplifiers have completely eliminated these problems. The PGA discrete circuit is shown in Figure 67. In Figure 67, when the 10pa bias current of the AD8610 drops at the (<5Ω) r of the switch, it results in a negligible offset error.

As shown in the circuit in Figure 67, the error introduced by the PGA is within the 1/2 LSB requirement of a 16-bit system when using high precision resistors.

1. Calculation theory B of room temperature error caused by R and I:

ΔVOS = IB × RON = 2 pA × 5 Ω = 10 pV

Total Offset = AD8610 (Offset) + ΔVOS

Total Offset = AD8610 (Offset_Trimmed) + ΔVOS

Total Offset = 5 μV + 10 pV ≈ 5 μV

2. Calculation theory B of total temperature error caused by r and i:

ΔVOS (@ 85°C) = IB (@ 85°C) × RON (@ 85°C) = 250 pA × 15 Ω = 3.75 nV

3. The temperature coefficient of the switch combined with AD8610/AD8620 is basically the same as the TV of AD8610/AD8620:

ΔΔVOS/ΔT(total) = ΔVOS/ΔT(AD8610/AD8620) + ΔVOS/ΔT(IB × RON)

ΔVOS/ΔT(total) = 0.5 μV/°C + 0.06 nV/°C ≈ 0.5 μV/°C

High Speed Instrumentation Amplifier

The 3-op amp instrumentation amplifier shown in Figure 68 can provide gains ranging from unity to 1000 or more. The instrumentation amplifier configuration features high common-mode rejection, balanced differential inputs, and stable, precisely defined gain. The JFET input AD8610/AD8620 achieve low input bias current and fast settling. Most instrumentation amplifiers cannot match the high frequency performance of this circuit. The circuit has a bandwidth of 25 MHz with a gain of 1 and is close to 5 MHz with a gain of 10. Settling time for the entire circuit is 550ns to 0.01% in 10V steps (gain = 10). Note that the resistor values around the input pins must be small enough so that their combined rc time constant with stray circuit capacitance does not degrade the circuit bandwidth.

high speed filter

The four most popular configurations are Butterworth, Oval, Bessel (Thompson) and Chebyshev. Each type has a response optimized for a given characteristic.

In active filter applications using op amps, the DC accuracy of the amplifier is critical for optimal filter performance. The bias voltage and bias current of the amplifier are the main sources of output error. The input bias voltage is passed through a filter and can be amplified to produce excessive output bias. For low frequency applications that require large value input resistors, the bias and bias currents flowing through these resistors also generate bias voltages.

At higher frequencies, the dynamic response of the amplifier must be carefully considered. In this case, slew rate, bandwidth, and open-loop gain play an important role in amplifier selection. The slew rate must be fast and symmetrical to minimize distortion. The bandwidth of the amplifier and the gain of the filter together determine the frequency response of the filter. Using high performance amplifiers such as the AD8610/AD8620 minimizes dc and ac errors in all active filter applications. Second-Order Low-Pass Filter Figure 69 shows the AD8610 configured as a second-order Butterworth low-pass filter. As shown, the design corner is 1 MHz and the bench measurement is 974 kHz. The wide frequency band of the AD8610/AD8620 allows corner frequencies into the megahertz range, but the input capacitance should be taken into account by making C1 and C2 smaller than the calculated values. The following equations can be used for part selection:

High Speed Low Noise Differential Drive

The AD8620 is ideal for low noise differential drivers for many popular ADCs. There are other applications (such as balanced lines) that require differential drivers. The circuit of Figure 70 is a unique line driver widely used in industrial applications. Using a ±13 V supply, the line driver can input a differential signal of 23 V PP into a 1 kΩ load. The high slew rate and wide frequency band of the AD8620 produces a full power bandwidth of 145 kHz at low noise in the front end producing a reference input noise voltage spectral density of 6 nV/√Hz. The design is a balanced transmission system. Transformerless, where output common-mode noise rejection is paramount. Just like in transformer designs, either output can be shorted to ground unbalanced. Line driver applications do not change the circuit gain of 1. This makes it easy to set the design to not flip, invert, or operate differentially.